Question 625401
First problem:  sqrt 15b * sqrt 3b= sqrt 45b^2.  You can simplify this to 3b sqrt 5, because 9 times 5 is 45, and 9 is a perfect square.  So, sqrt of 9 is 3 and you leave that outside of the radical sign.  Sqrt of b^2 is b, so you leave that outside of the radical sign. 5 is not a perfect square, you put that under the radical sign.  So, 3b sqrt 5.
Plug in a same number to both equations, you should get the same answer!

The second problem. (2-sqrt6)(1+sqrt6).  Use FOIL to solve this.  So, 2*1 is 2. 2*sqrt6 is 2sqrt6.  -sqrt6*1 is  -1sqrt6.  -sqrt6*sqrt6 is -6, since the radical sign cancel outs.  So, then answer is -4+1sqrt6.